This invention relates to decorative and reliable fasteners, and particularly automobile wheel lug nuts and bolts.
By way of background, lug nuts and lug bolts are commonly used to attach wheels to the axles of automobiles and other vehicles. Lug nuts are most commonly found on American automobiles whereas European automobiles typically employ lug bolts. In a lug nut mounting configuration, the axle rotor or drum to which the wheel is mounted has protruding externally threaded studs. The wheel hub has openings that fit over the studs, and the lug nuts thread onto the studs to hold the wheel in place. In a lug bolt mounting configuration, the rotor or drum has internally threaded openings. The wheel hub also has openings that align with the threaded openings. The lug bolts extend through the wheel hub openings and engage the threaded openings to secure the wheel.
Because lug bolts and nuts must operate in highly corrosive environments, they are usually coated in their entirety with a protective coating. The coating may be decorative or non-decorative. Many decorative coatings, such as those comprising nickel-chrome plating or the like, are highly corrosion resistant and render the fastener more durable in its environment. There are, however, two problems associated with using a decorative coating on a fastener: (1) the decorative coating does not allow a proper torque-tension relationship to develop between the fastener and the wheel hub and (2) such a coating usually deposits non-uniformly on the threaded portion of the fastener, enlarges the threads, and reduces their mechanical holding power. Both conditions tend to alter the retention torque between the nut seat and the wheel hub seat or between the bolt seat and the wheel hub seat. The altered torque may allow the nuts or bolts to loosen. In the worst case, the wheel could fall off the axle.
As an alternative to decorative coatings, manufacturers often coat fasteners with non-decorative coatings. Such coatings, which are typically zinc-based, normally deposit uniformly on the fastener threads and provide the proper torque-tension relationship between the fastener and the wheel hub. More specifically, they provide a soft, sacrificial, cathodic metal that acts as a solid lubricant. As indicated, such coatings are typically zinc-based. Zinc is a well known metal that is easily and inexpensively coated onto a ferrous substrate to galvanize the substrate and provide corrosion and cathodic protection and lubricity. Zinc naturally acquires a protective oxide when it is exposed to air. It therefore resists rust when it is coated on a ferrous product such as a lug bolt or lug nut. When the zinc coating is scratched to expose the underlying metal, the zinc will oxidize before the metal and cover the metal with a zinc oxide coating to prevent oxidation of the metal. Zinc also has a relatively low coefficient of friction and is the preferred coating of choice for lug bolts and lug nuts insofar as it acts as a solid lubricant that assists in tightening the bolts or nuts.
A deficiency of zinc-based and other non-decorative coatings is that they fail to provide the desired aesthetic appearance and usually lack the corrosion-resistant properties of decorative coatings. Moreover, repeated exposure to chemical solvents, such as those used at commercial car wash establishments, can remove a non-decorative coating from the exposed portion of a fastener.
As a result of the foregoing, some manufacturers have resorted to two-piece solutions in which a decorative cap is mounted over the head portion of a fastener that has been coated with a non-decorative coating. Exemplifying this approach are U.S. Pat. Nos. 5,395,196, 5,370,486, 5,180,266, 4,775,272, and 4,018,133. In the designs disclosed in the referenced patents, the decorative caps are pressed, welded or otherwise joined to the non-decoratively coated fastener. The resultant two-piece system provides the mechanical clamping (torque-tension) characteristics of the non-decoratively coated fastener while providing the decorative characteristics of the decorative cap. However, these systems are relatively expensive to manufacture, can suffer from galvanic corrosion at the contacting surfaces of the cap and the fastener (which are of different material), and may lack 100% geometric conformance between the cap and the fastener. The latter two deficiencies can lead to cracks and voids. Weather and mechanical vibration, over time, may even cause the cap to separate from the fastener.
Accordingly, there is a long-standing unmet need for fasteners, and particularly automobile lug nuts and bolts, that are both aesthetically pleasing and mechanically reliable. What is needed is a fastener that provides an acceptable appearance where required, a proper torque-tension relationship between the work piece and the fastener, and a durable, easy to manufacture product.
The foregoing problems are solved, and an advance in the art is provided, by a one-piece fastener with two distinct coatings applied over selected portions of the fastener. One coating is decorative and corrosion resistant. The other coating is optimized for the desired mechanical holding power. The invention can be made in the form of a bolt or a nut. In the preferred embodiment of the inventive bolt, the bolt has a head and a shank. The head represents an exposed portion of the bolt and is covered with a first decorative/corrosion-resistant coating. The shank includes a tapered seating surface and an elongated threaded portion that passes through a threaded aperture in a wheel axle. The seating surface and the threaded portion of the shank represent an unexposed portion of the bolt. They are covered with a second coating that provides a proper torque-tension relationship between the seating surface and the wheel hub, and between the threaded portion of the shank and the threaded aperture in the wheel axle. The first and second coatings are mechanically distinct from each other.
In the preferred embodiment of the inventive nut, the nut has polygonal sides for engaging a force-applying member, a top surface, a tapered seating surface, and a threaded bore. The polygonal sides and the top surface represent an exposed portion of the nut. They are covered with a first decorative/corrosion-resistant coating. The seating surface and the threaded bore represent an unexposed portion of the nut. They are covered with a second coating that provides a proper torque-tension relationship between the seating surface and the wheel hub, and between the threaded bore and a stud extending from the wheel axle.
The fastener may also be another type of fastener, such as a bolt-lock structure or a lock nut. A color coating can be used to associate the bolt-lock or lock nut with a force-applying member (such as a wrench) having a mating locking key.
In a further aspect of the invention, a fastener such as a bolt or a nut is coated with an auxiliary coating over all or part of its unexposed portion prior to application of the above-mentioned second coating. The auxiliary coating is preferably different from the second coating. The first coating is applied over the exposed portion of the fastener and the second coating is applied over the entirety of the unexposed portion of the fastener, including the parts that were coated with the auxiliary coating. The foregoing configuration has certain processing advantages. Moreover, the auxiliary coating serves to protect the fastener in areas where the second coating could be removed by solvents or other environmental agents.